1. Field of the Invention
The present invention is directed to circuitry utilized in an LED indicator, such as a LED traffic signal, which improves safety of the LED indicator by ensuring that it is on and off at appropriate times.
2. Discussion of the Background
Utilizing light emitting diodes (LEDs) as indicators is common because, among other things, LEDs provide benefits of long life and low power consumption over conventional indicators such as incandescent lamps, fluorescent lamps, etc. Traffic signals utilizing an array of LEDs as an indicator are also known.
Traffic signals are generally powered from controllers located at each traffic intersection. Those controllers have load switches, which are typically solid state relays, that switch power on and off to the traffic signals. For safety reasons the controllers also include conflict monitor circuitry which is utilized to ensure safe operation of the traffic signal, among other things. More particularly, the conflict monitor circuitry seeks to prevent unsafe situations such as a light failing to illuminate or multiple lights illuminating simultaneously. To perform those functions the conflict monitor circuitry must determine for each traffic signal at the intersection which lights are on and which lights are off.
Conflict monitor circuitry senses voltage across a traffic signal light to determine if it is lit or not. This circuitry is typically set to comply with NEMA standards for traffic signal controllers. For red lights, the conflict monitor circuitry treats voltages less than 50 volts as "off", 50-70 volts as "undefined", and greater than 70 volts as "on". Green and yellow lights are treated in similar fashion except at different voltages. They are treated as "on" for voltages of 25 volts or higher, undefined for 15-25 volts, and "off" for voltages less than 15 volts.
In the context of a red traffic light, it is most important that the conflict monitor circuitry accurately determine whether the red light is on. Otherwise, vehicle motorists could mistakenly enter an intersection when cross traffic is authorized, creating an unsafe condition. The conflict monitor circuitry exists to prevent this scenario, switching to a flashing mode when measured voltages imply no light or dual indication.
For the green or yellow lights the same voltage characteristic is measured. However, the most important characteristics for the green and yellow lights is that if the conflict monitor circuitry interprets the green and yellow lights as being off, that the green and yellow lights in fact be off. Otherwise, it is possible that both green and red signals are lit simultaneously, causing confusion and a possible safety hazard.
Even though line voltages for traffic signals are typically 120 VAC, it is possible for traffic signals to experience voltage potentials across them between 25 to 50 VAC, at which point the conflict monitor circuitry may not have a clear indication as to whether certain lights are on or off. Such a situation may also occur with incandescent lamps.
A potentially dangerous situation may arise when traffic signals are connected in a particular configuration which includes a high-impedance connection between an AC input neutral connection of two signals. Such a situation is further explained with reference to FIG. 1.
FIG. 1 shows a situation in which two traffic signals are connected to an input line voltage of 120 VAC and a neutral signal line. Load switches S.sub.R, S.sub.Y, and S.sub.G for each color light are utilized to turn lights on and off.
In the situation shown in FIG. 1, the red traffic lights R.sub.1 and R.sub.2 are to be turned on and illuminate when switch S.sub.R is closed. In the instance when these red lights R.sub.1 and R.sub.2 are to illuminate, current flows through red lights R.sub.1 and R.sub.2 from the 120 VAC input line voltage to the neutral lines N.sub.1 and N.sub.2. However, if a high impedance 7 exists between the neutral line N.sub.1 from one signal head and its connection with the other neutral line N.sub.2, then the current may be diverted in a reverse path through green traffic signal G.sub.1 and yellow signal Y.sub.1, as shown by the arrows in FIG. 1. The current then flows in a normal direction through the green traffic light G.sub.2 and yellow signal Y.sub.2 to the output on the neutral line N.sub.2 to complete the current flow to the AC line. That is, a situation may arise in which high impedance 7 exists between the two common connections. The high impedance 7 is formed by a fault in the wiring connecting the lights. In such a situation as shown in FIG. 1, the green G.sub.1 and G.sub.2 and/or yellow Y.sub.1, and Y.sub.2 lights may appear somewhat illuminated at the same time as the red lights R.sub.1 and R.sub.2. In that situation, a red light and a green light facing in a same direction may be illuminated simultaneously, causing a dangerous situation as an oncoming motorist will not know whether to proceed or stop at the traffic intersection.